Vacuum processing chambers are generally used for chemical vapor depositing (CVD) of materials on substrates by supplying process gas to the vacuum chamber and applying a radio frequency (RF) field to the gas. A number of gas distribution systems for integrated circuit processing are known, but the vast majority of known systems are designed for low-density, high pressure plasma etching or for plasma enhanced CVD (PECVD). Conventional gas distribution systems typically deliver reactants at relatively low flow rates. Showerhead gas injection and diffusive transport systems are commonly used to ensure even distribution over the substrate.
These known systems are not optimized for high density plasma CVD (HDPCVD) processes, such as encapsulation and intermetal dielectric gap filling. In HDPCVD it is important to focus the delivery of reactants such as silane related species onto a substrate, because silane and its fragments, e.g., SiH.sub.3, SiH.sub.2, SiH, and so on, have high sticking coefficients. Directing the silane preferentially onto the substrate is advantageous because it maximizes the substrate deposition rate and minimizes film deposits on various internal surfaces of the reactor.
There are various known systems for increasing the concentration of process gas above the substrate surface. For example, FIG. 1 shows a system including a plasma source 110 for generating a plasma in a chamber 140 and a gas ring 167 with attached gas inlets supplying process gas into the processing chamber 140 for processing a substrate 120 on a substrate support 130. Gas is supplied to the plenum 167a of gas ring 167 via a gas delivery port 167b from a gas source (not shown). This type of system may also include an additional gas ring 160. Gas is supplied to the plenum 160a of the gas ring 160 through a gas delivery port (not shown). Conventionally, the deposition rate in such a system is increased by concentrating the process gas above the substrate 120. This is typically done by changing the distance from the gas ring 167 to the substrate 120. The more the process gas is concentrated toward the area above the center of the substrate, the larger the peak deposition rate. Unfortunately, in concentrating the process gas near the center of the substrate, the deposition rate on the outer portion of the substrate may not increase as much as the center, which leads to a potential decrease in deposition uniformity.
U.S. Pat. No. 4,691,662 to Roppel et al. discloses a dual plasma microwave apparatus for etching and deposition in which process gas is fed by conduits mounted on a side wall of a processing chamber, extending over a portion of the substrate. U.S. Pat. No. 5,522,934 to Suzuki et al. discloses a gas injector arrangement including a plurality of gas supply nozzles positioned in a plurality of levels in a direction substantially perpendicular to the substrate. The gas supply nozzles at upper levels extend further toward the center of the substrate than those at lower levels. The injection holes are located at the distal ends of the gas supply nozzles. These systems are effective in delivering the process gas to the region above the substrate. However, because the conduits extend over the substrate surface between the substrate and the primary ion generation region, as the ions diffuse from the generation region toward the substrate the conduits can cast shadows of ion nonuniformity onto the substrate surface. This can lead to an undesirable loss in etch and deposition uniformity.
Other approaches employ gas supply conduits which do not extend over the substrate surface. "Electron Cyclotron Resonance Microwave Discharges for Etching and Thin-film Deposition," J. Vacuum Science and Technology A, Vol. 7, pp. 883-893 (1989) by J. Asmussen shows conduits extending only up to the substrate edge. "Low-temperature Deposition of Silicon Dioxide Films from Electron Cyclotron Resonant Microwave Plasmas," J. Applied Physics, Vol. 65, pp. 2457-2463 (1989) by T. V. Herak et al. illustrates a plasma CVD tool including a plurality of gas injection conduits which feed separate process gases. One set of conduits is mounted in the lower chamber wall with gas delivery orifices located just outside the periphery of the substrate support and at the distal ends of the conduits. "New Approach to Low Temperature Deposition of High-quality Thin Films by Electron Cyclotron Resonance Microwave Plasmas," J. Vac. Sci. Tech, B, Vol. 10, pp. 2170-2178 (1992) by T. T. Chau et al. illustrates a plasma CVD tool including a gas inlet conduit mounted in the lower chamber wall, located just above and outside the periphery of the substrate support. The conduit is bent so that the injection axis is substantially parallel to the substrate. An additional horizontal conduit is provided for a second process gas. The gas injection orifices are located at the distal ends of the conduits. A problem with all of these gas injection plasma processing devices is that gas is injected from the distal ends of the conduits. Injectors with the orifices located at the distal ends of the injector tubes may be prone to clogging after processing a relatively small batch of substrates, e.g., less than 100. This injector orifice clogging is detrimental as it can lead to nonuniform distribution of reactants, nonuniform film deposition or etching of the substrate, and shifts in the overall deposition or etch rate.
Various systems have been proposed to improve process uniformity by injecting process gas at sonic or supersonic velocity. For example, U.S. Pat. No. 4,270,999 to Hassan et al. discloses the advantage of injecting process gases for plasma etch and deposition applications at sonic velocity. Hassan et al. notes that the attainment of sonic velocity in the nozzle promotes an explosive discharge from the vacuum terminus of the nozzle which engenders a highly swirled and uniform dissipation of gas molecules in the reaction zone surrounding the substrate. U.S. Pat. No. 5,614,055 to Fairbairn et al. discloses elongated supersonic spray nozzles which spray reactant gas at supersonic velocity toward the region overlying the substrate. The nozzles extend from the chamber wall toward the substrate, with each nozzle tip having a gas distribution orifice at the distal end. U.S. Pat. No. 4,943,345 to Asmussen et al. discloses a plasma CVD apparatus including supersonic nozzles for directing excited gas at the substrate. U.S. Pat. No. 5,164,040 to Eres et al. discloses pulsed supersonic jets for CVD. While these systems are intended to improve process uniformity, they suffer from the drawbacks noted above, namely clogging of the orifices at the distal ends of the injectors, which can adversely affect film uniformity on the substrate.
U.S. Pat. No. 4,996,077 to Moslehi et al. discloses an electron cyclotron resonance (ECR) device including gas injectors arranged around the periphery of a substrate to provide uniform distribution of non-plasma gases. The non-plasma gases are injected to reduce particle contamination, and the injectors are oriented to direct the non-plasma gas onto the substrate surface to be processed.
U.S. Pat. No. 5,252,133 to Miyazaki et al. discloses a multi-wafer non-plasma CVD apparatus including a vertical gas supply tube having a plurality of gas injection holes along a longitudinal axis. The injection holes extend along the longitudinal side of a wafer boat supporting a plurality of substrates to introduce gas into the chamber. Similarly, U.S. Pat. No. 4,992,301 to Shishiguchi et al. discloses a plurality of vertical gas supply tubes with gas emission holes along the length of the tube. These patents relate to thermal, non-plasma CVD, and are thus not optimized for plasma processing.
As substrate size increases, center gas injection is becoming increasingly important for ensuring uniform etching and deposition. This is particularly evident in flat panel display processing. Typically, diffusive transport is dominant in the region above the substrate in these low pressure processing systems, while convective transport plays much less of a role. Near the injection orifices, however, convective transport can dominate diffusive transport because of the jet-like nature of the injected gas. Locating the injection orifices closer to the substrate therefore increases the convective transport in relation to the otherwise dominant diffusive transport above the substrate. Conventional showerhead gas injection systems can deliver gases to the center of the substrate, but in order to locate the orifices close to the substrate, the chamber height must be reduced which can lead to an undesirable loss in ion uniformity.
Radial gas injection systems may not provide adequate process gas delivery to the center of large area substrates typically encountered, for example, in flat panel processing. This is particularly true in bottom-pumped chamber designs commonly found in plasma processing systems. Without a means for center gas feed, etch byproducts may stagnate above the center of the substrate, which can lead to undesirable nonuniform etching and profile control across the substrate.
The above-mentioned Fairbairn et al. patent also discloses a showerhead injection system in which injector orifices are located on the ceiling of the reactor. This showerhead system further includes a plurality of embedded magnets to reduce orifice clogging. U.S. Pat. No. 5,134,965 to Tokuda et al. discloses a processing system in which process gas is injected through inlets on the ceiling of a processing chamber. The gas is supplied toward a high density plasma region. This system employs microwave energy and is not optimized for radio frequency plasma processing. U.S. Pat. No. 5,522,934 to Suzuki et al. disclose a system where inert (rather than process) gas is injected through the center of the chamber ceiling.
There is thus a need for optimizing uniformity and deposition for radio frequency plasma processing of a substrate while preventing clogging of the gas supply orifices and build up of processing by-products and improving convective transport above the wafer.